Add test for issue-44861

Author: JohnTitor

src/test/ui/specialization/issue-44861.rs

@@ -0,0 +1,40 @@
+#![crate_type = "lib"]
+#![feature(specialization)]
+#![feature(unsize, coerce_unsized)]
+#![allow(incomplete_features)]
+
+use std::ops::CoerceUnsized;
+
+pub struct SmartassPtr<A: Smartass+?Sized>(A::Data);
+
+pub trait Smartass {
+    type Data;
+    type Data2: CoerceUnsized<*const [u8]>;
+}
+
+pub trait MaybeObjectSafe {}
+
+impl MaybeObjectSafe for () {}
+
+impl<T> Smartass for T {
+    type Data = <Self as Smartass>::Data2;
+    default type Data2 = ();
+    //~^ ERROR: the trait bound `(): std::ops::CoerceUnsized<*const [u8]>` is not satisfied
+}
+
+impl Smartass for () {
+    type Data2 = *const [u8; 1];
+}
+
+impl Smartass for dyn MaybeObjectSafe {
+    type Data = *const [u8];
+    type Data2 = *const [u8; 0];
+}
+
+impl<U: Smartass+?Sized, T: Smartass+?Sized> CoerceUnsized<SmartassPtr<T>> for SmartassPtr<U>
+    where <U as Smartass>::Data: std::ops::CoerceUnsized<<T as Smartass>::Data>
+{}
+
+pub fn conv(s: SmartassPtr<()>) -> SmartassPtr<dyn MaybeObjectSafe> {
+    s
+}

src/test/ui/specialization/issue-44861.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the trait bound `(): std::ops::CoerceUnsized<*const [u8]>` is not satisfied
+  --> $DIR/issue-44861.rs:21:5
+   |
+LL |     type Data2: CoerceUnsized<*const [u8]>;
+   |     --------------------------------------- required by `Smartass::Data2`
+...
+LL |     default type Data2 = ();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::ops::CoerceUnsized<*const [u8]>` is not implemented for `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.